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DPG Tagung, O27, Phasenübergänge, Berlin 2005 Surface freezing and surface phase transition studied at the liquid eutectic AuSi surface 1. Surface layering.

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Presentation on theme: "DPG Tagung, O27, Phasenübergänge, Berlin 2005 Surface freezing and surface phase transition studied at the liquid eutectic AuSi surface 1. Surface layering."— Presentation transcript:

1 DPG Tagung, O27, Phasenübergänge, Berlin 2005 Surface freezing and surface phase transition studied at the liquid eutectic AuSi surface 1. Surface layering at liquid/vapor interface of metals/alloys 2. Surface freezing of AuSi above bulk melting temperature 3. Solid-solid surface phase transition Reinhard Streitel, Oleg Shpyrko, Alexei Grigoriev Supervisor: Prof. P. Pershan, DEAS, Harvard University

2 Surface-Induced Layering Disordered Interface (classical Van-der-Waals description 1873) Ordered Interface Surface-Induced Layering (perdicted 1984, S. A. Rice) Liquid-vapor interface

3 Layering observed in liquid metals and alloys Experimental measurements (NSLS & APS): Simple Liquids (Pershan Group, 1995-2004) Gallium, Mercury, Indium, Potassium, Tin Binary Liquids (Pershan, Rice, Freyland groups;1997-2004) GaIn, BiIn, GaPb, BiPb, HgAu, GaBi, KNa, AuGe, BiSn, AuSi Scattering geometry layering peak: (a atomic radii) a ~ 1.0–1.5Å q z peak =  /a q z peak ~ 2.0-3.0 Å -1

4 Reflectivity of eutectic AuSi (1) Eutectic composition Au 0.81 Si 0.19 (atomic %) T M (bulk) = 361-363°C Phase diagram AuSiReflectivity @ different temperatures: @ T =371°C (about 10 °C above T M (bulk) !) change in reflectivity for high q z T M (Au)=1063°C T M (Si)=1410°C Transition at T M + 10 °C

5 Surface-Normal (Layering) Structure Normalized X-ray Reflectivity R/R F Electron Density Profile Electron Density z, Angstroms - Reflectivity and structure related through Fourier Transform - Width / Height of quasi-Bragg peak determined by layering decay length

6 Reflectivity of eutectic AuSi (2)

7 Surface Phase Transition at 371 o C First-Order Transition Reversible in Temperature

8 In-Plane Structure: X-GID Surface freezing and surface phase transition

9 Surface freezing - Surface freezes above bulk melting point (!): 2D solid surface layer coexists with liquid bulk phase - Surface freezing in other systems e.g. dilute Pb on liquid Ga (S. A. Rice) BUT surface and bulk chemically different S.A. Rice et al., Phys. Rev. B 62, 13111-13120 (2000) liquid Ga Pb

10 Structure model: low temperature phase AuSi 2 7.4 C 9.4 C 12 atoms 4 Au, 8 Si Au Si

11 Summary surface layering observed in liquid metals and alloys liquid AuSi eutectic shows temperature dependence in reflectivity for high q z a reversible solid-solid phase transition occurs at about 10 °C above bulk melting temperature 2D AuSi 2 structure describes high temperature phase GID peaks future: reflectivity at even higher temperature understanding details of structural changes

12 Acknowledgments For more info visit http://liquids.deas.harvard.eduhttp://liquids.deas.harvard.edu Harvard: Prof. Peter Pershan Oleg Shpyrko Alexei Grigoriev CARS, U. Chicago: Jim Viccaro, Binhua Lin Tim Graber, Mati Meron Jeff Gebhardt Ben Ocko Prof. Mosche Deutsch Research was supported by US DOE grant (DE-FG02-88-ER45379). The work was performed in ChemMatCARS sector of Advanced Photon Source, Argonne National Lab. ChemMatCARS at the APS is supported by NSF/ DOE grant number CHE0087817. Bar-Ilan, Israel: Brookhaven:

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